As shown in FIG. 7, a gas sensor 9 is known which inserting a sensor element 911 into first insulation porcelain 912 assembles and then mounting the first insulation porcelain 912 in a housing 913.
The sensor element 911 has a measuring electrode exposed to measurement gas and a reference electrode exposed to reference gas. The sensor element 911 determines a concentration of a particular gas by providing a signal in the form of an ion current flowing through the measuring electrode and the reference electrode or of a potential difference between the measuring electrode and the reference electrode.
A gas leak from a measurement gas chamber 915 to an air chamber 914 will result in a decrease the accuracy of measuring the concentration of the particular gas.
In Japanese Patent Laid-open Publication No. 2002-82085, it is described that the gas sensor 9 has the first insulation porcelain 912 carried on the housing 913 via a metal packing ring 92.
As shown in FIGS. 7 and 8, a seat surface 917 formed on an inside surface of the housing 913 supports a contacting surface 916 formed on an outside surface of the first insulation porcelain 912 via the metal packing ring 92. Specifically, a gap between the seat surface 917 of the housing 913 and the contacting surface 916 of the first insulation porcelain 912 is sealed hermetically by the metal packing ring 92 to keep the air chamber 914 and the gas chamber 915 airtight.
More recently, however, since engines are required to have lower fuel consumption and higher output power for protecting the environment, the temperature of the exhaust gas has increased. When the gas sensor 9 is exposed to the high temperature, the temperature of the metal packing ring 92 rises, too. After the gas sensor 9 is used many times, as shown in FIG. 8, an oxidized corrosion 920 becomes in the whole metal packing ring 92 and then the durability of the metal packing ring 92 deteriorates. Therefore, there is the concern that the gap between the air chamber 914 and the gas chamber 915 can not be sufficiently hermetically sealed by the metal packing ring 92.